In recent years, ink-jet heads which discharge various kinds of ink, such as electrically conductive ink, have been put to practical use. In such ink-jet heads, it is necessary to protect an electrode, etc. from ink. In addition, in an ink-jet head which discharges special kind of ink, such as solvent ink, there arises such a problem that an adhesive, which attaches a nozzle plate and a piezoelectric member, is degraded by the ink. It is necessary, therefore, to protect those parts of the ink-jet head, which have poor ink resistance properties.
To meet such a demand, techniques have been studied for coating an electrode, a nozzle plate, etc. with a protection film which is formed of a high-molecular-weight material. However, when a growth variance of a protection film has occurred in an edge portion of a nozzle hole in the nozzle plate, there may occur such a problem that a variance also occurs in ink discharge performance.
If the protection film has abnormally grown at the edge portion of the nozzle hole, the abnormal growth point (projection) adversely affects ink drops at the time of ink discharge, leading to a decrease in print quality. For example, if the direction of discharge of an ink drop is inclined, an error occurs in the position of a dot which is formed on a medium by the ink drop. In addition, in the case where a main ink drop, which is to be discharged, has trailed and a small ink drop (satellite) has occurred, the satellite may fly in a direction different from the direction of the main ink drop, and a small dot, which is formed by the small ink drop, may be printed on the medium in addition to the main dot which is formed by the main ink drop.